1. Field of the Invention
The present invention relates to a method and apparatus which uses a solvatochromic probe molecule admixed in a polymerizable composition in order to determine when a cured polymer is produced. There is a wavelength or intensity shift in the fluorescence spectrum of the probe molecule in the cured polymer as compared to the uncured polymer. In particular, the present invention relates to a method wherein a ratio of the fluorescence intensity of the probe molecule in the polymerizable composition to the fluorescence intensity of the probe molecule in the cured polymer is determined at different wavelengths related to the shift in fluorescence spectrum produced upon curing.
2. Description of Related Art
In the field of polymer and composites processing, there is a compelling need for the development of inexpensive on-line sensors for determining viscosity and degree of cure. For example, the manufacture of polymer-matrix composites involves a series of complex chemical and physical changes which must be adequately controlled to produce products with desirable properties. Due to the lack of appropriate on-line sensors for properties such as viscosity and degree of cure, most quality control efforts are based on off-line measurements made after the product has completed its cure cycle. On-line monitoring sensors which could be interfaced with an electronic control scheme would significantly enhance the reliability of polymer and composites processing methods while reducing the cost.
The need for such sensors has been recognized widely, and the measurement of a variety of physical phenomena has been explored, including dielectric constant and loss, acoustic and ultrasonic wave propagation, and optical techniques ("Process Monitoring Sensors for Polymer Composites," U.S. Department of Commerce, Report #NISTIR 4514 (1991)). To date, none of these approaches has resulted in the development of an inexpensive on-line sensor.
Fluorescence techniques have been proposed for the development of on-line viscosity sensors due to the fact that the fluorescence behavior of a variety of molecular probes depends on the local viscosity. For example, many fluorescent probes exhibit an enhanced fluorescence intensity with increasing viscosity due to decreased non-radiative energy transfer. Levy and Schwab (Levy, R. L., and S. D. Schwab, Polymer Composites 12 96 (1991) and U.S. Pat. No. 5,158,720) and Wang et al. (Wang, F. W., et al., in "Photophysics of Polymers, edited by C. E. Hoyle and J. M. Torkelson, ACS, Washington, D.C. (1987)) demonstrated that such an increase in fluorescence intensity can be used to monitor the viscosity increase during cure. Another publication is Chapter 9 of ACS Symposium Series 367 (1988). The measurement of absolute fluorescence intensity, however, has some significant limitations. For example, the method has limited utility for monitoring the degree of cure above the gel point (limited dynamic range) and is subject to uncertainty due to variations in background fluorescence. Stroeks et al. (Stroeks, A., et al., Polymer 29 467 (1988)) investigated the use of viscosity-sensitive excimer formation to monitor degree of cure. In addition to the aforementioned limitations of the previous technique, the investigators concluded that apparent intensity changes in the excimer peak were actually dominated by changes in the intensity of the overlapping monomer peak (which was increasing because of reduced non-radiative transfer). Finally, Scarlata and Ors (Scarlata, S. F., et al., Polymer Comm. 27 41 (1986)) have monitored an increase in fluorescence polarization which they attribute to a viscosity-induced decrease in the rotational mobility of the probe molecule. However, due to its complexity, this technique is unlikely to form the basis for an inexpensive on-line sensor for polymer and composites processing due to its need to measure absolute quantities. See also Tan, W. et al., Science 258, 778-781 (1992); Holtom, Gary R., ASTM Proceedings, 1204-01 (1990); Wang, F. W., ACS, 33 454-462 (1987).
Solvatochromic compounds are known to the prior art. Such compounds are described in U.S. Pat. No. 4,941,997 to Decher et al.; U.S. Pat. No. 5,068,318 to Decher; 4,722,983 to Monnier et al. and EP 353650 to Hayashi et al.
A number of compounds used in photopolymerization of vinyl monomers can also be solvatochromic, such as described in U.S. Pat. No. 5,137,800 to Neckers et al. U.S. Pat. Nos. 3,716,595, 3,557,218 and 3,728,394 to Hall et al. describes photochromatic dihydropyrenes. U.S. Pat. No. 3,551,344 to Gardlund et al. describes photochromatic polyaromatic compounds used in plastics. Some polymers are photoconductive.
Blanchard, G. J., Chemical Physics 138 365-375 (1989); Blanchard et al., J. Phys. Chem. 92: 5850-5854 (1988); and Karpovich and Blanchard, J. Phys. Chem., 99:3951-3958 (1995) describe various dyes which can be solvatochromic.
OBJECTS
It is therefore an object of the present invention to provide a novel method and apparatus for determining the degree of cure of a polymer. In particular it is an object of the present invention to provide a method which is relatively simple and economical. These and other objects will become increasingly apparent by reference to the following description and the drawings.